A search for far-infrared HCN lasers in C-rich evolved stars

The pumping mechanisms for astrophysical masers are often complex, involving infrared line opacities, collisions, and infrared line overlaps. Hence, the interpretation of the observed emission requires detailed modelling. In spite of these potential problems the masering lines can, however, give useful information on the physical conditions of the regions where the masers are produced, i.e., the innermost zones of circumstellar envelopes. While SiO, OH, and H2O masers are a common feature of Orich stars and were detected more than forty years ago, strong maser emission in Crich stars has been detected only in some vibrational linesof HCN, and in some highJ lines of the ground state of SiS.In the line survey of IRC+10216 with HIFI we have discovered several lasers involving the pair of vibrational levels 4v2/v2+v1. The strongestones are above 1 THz and carry information of the 13 stellar radii zone. Several masers have been also found in some lines of the v2 bending mode between 500 and 900 GHz.We would like to carry a systematic search for HCN masers in Crich evolved stars by observing selected frequency ranges covering from the v1 to the 3v2 vibrational states (typically a range of 1520 GHz) for each rotational line within the SIS receivers of HIFI (from J65 up to J1312). The pummping mechanism can be different in each objectdepending on the dust opacity. High mass loss rate stars will havetheir 3 micron emission absorbed near the star by the large amount ofdust produced in these objects. The pumping of HCN will be done mainlythrough the bending mode at 14 microns. However, in low mass loss ratethe 3 micron photons will populate the v1, v3 and combination bands allowinga much complex pumping pattern.This study will permit to characterize HCN laser emission in Crich stars and their pumping mechanisms. It will also complement our recently granted high angular resolution observations of HCN with ALMA in cycl truncated! [truncated!, Please see actual data for full text]